1. Field of the Invention
The present invention relates to a method for controlling the temperature of a mold for an injection molding and more particularly, to a method for controlling the temperature of a mold for an injection molding for restraining the generation of a weld line and a chromatic aberration at the position where two melted and separated resins flow together, by separately controlling the temperature of each region of the mold.
2. Description of the Related Art
Generally, among the molding conditions of the injection molding, injecting pressure, injecting velocity, weighing and the temperature of the injector can be controlled using a controller mounted on the injector. However, the temperature of the mold should be separately and externally controlled. Otherwise, the control of the temperature of the mold is difficult.
At this time, when the temperature of the mold is controlled simply by cold water, the temperature of the mold becomes different on each region of the mold because of the change of the external environment and the flow of the melted resin in the cavity. Therefore, a continuous working for manufacturing a good product could not be accomplished. Further, even though the continuous working is not required, the molding condition should be frequently changed for the stability of the products. If the temperature of the mold is not constantly controlled, the standardization of the molding condition could not be accomplished due to the change of the injecting pressure, injecting velocity, etc.
Even though the size of the mold is dependent on the size of the mould product, initial defect may be generated during increasing the temperature of the mold from the low temperature state of the mold to the temperature of the mold where the good product can be obtained. As a result, the establishment of the standard molding condition is difficult.
For solving the above-mentioned problem, generally the injecting pressure is increased. In this case, the temperature of the mold increases to some degree according to the increase of the injecting pressure, however, burr of the mould product at the parting line may be generated.
FIG. 1 illustrates the relation of the temperature of the mold with respect to the distribution of the number of defects. Region A corresponds to the portion at which the molding is not completed and so a compression defect is generated. Region B corresponds to the medium portion of the good product and the defect. At region B, a crack may be generated owing to the defect of the weld line and this portion affects the reliability of the product. For minimizing this region, the mold is heated in advance through implementing a preheating process for stabilizing the temperature of the mold.
It is important to keep the temperature of the mold at a constant degree through heating as described above, however, it also is important to cool the mold for the cycle time of the mold.
The controlling of the temperature of the mold plays an important role for improving the productivity of the product and also plays an important role for generating the defect of the product.
That is, when the temperature of the mold is not homogeneous, a compression defect is generated at a boss portion and/or a rib portion where compression degrees are great. And when the temperature of the mold is too low, the molding is not completed or a silver streak defect is generated. Further, a defect of deflection deformation also is generated owing to the regional difference of the mold temperature.
Among the defects, the weld line generated on the surface of the product, is formed as a thin solid line at the portion where two melted and separated resins flow together in the mold. The main reason of the generation of the weld line is the incomplete mixing of the two separated and melted resins at the meeting portion because of the low temperature of the front end portions of two resins. The weld line also can be generated by moisture or a volatile matter contained in the resin, or by a mold release used during the injection molding.
Particularly, the generation of the weld line is caused when a hole is present in a mould product or two gates are present in the mold. For carrying out the injection molding, the gate for passing the resin is needed. And when the mould product is large or complicated, two or more gates are needed. Accordingly, the weld line is liable to be formed. Even though the melted resin is supplied through one gate, when an inserting component or a hole is present in the mould product or when the building thickness of the mould product is different, the weld line may be generated.
FIGS. 2A and 2B are typical views for explaining the generating process of the weld line. The melted resin is supplied into the cavity through two gates 12 and 14 and a weld line 16 is generated where the two separated portions of the resin flow together as shown in FIG. 2A.
At this time, the weld line is not formed even though the separated and melted resins flow together near the gates 12 and 14 but the weld line is formed far from the gates 12 and 14. The melted resin gets cool through the mold and the two portions far from the gates 12 and 14 could not mix together completely because of the low temperature of the resins, thereby generating the weld line 16.
Meanwhile, even though the melted resin is supplied through one gate 12, the weld line 16 is generated when a hole 18 is provided in the mould product because the melted resin is separated around the hole 18 and then flow together, as shown in FIG. 2B.
Thus formed weld line deteriorates the exterior view of the mould product and therefore, a painting process for eliminating the weld line is additionally implemented after the injection molding. This makes the working process complicated and increases the manufacturing cost.